A monitor of chord-averaged plasma density

A single-channel two-frequency chord-averaged density monitor for measuring the plasma density in tokamaks is described. The operating principle of the monitor is based on measuring the phase shift between two microwave signals at close frequencies transmitted through plasma along one and the same chord. At an appropriately chosen difference between these frequencies, the measured phase difference is <2π in the entire range of measured densities, thereby ensuring measurements without fringe jumps. The advantage of this method for measuring the delay time compared to the pulsed method consists in an increased sensitivity of measurements owing to the possibility of filtering signals at fairly high intermediate frequency (∼0.5–1 GHz). It is additionally possible to adjust the dynamic range of density measurements by selecting (optimizing) the frequency difference. The schematic diagram and the main performance characteristics of the monitor are presented. The results from laboratory tests of the monitor and those obtained with it on the T-11M facility are presented. The range of density measurements is 0.7 × 1012–0.7 × 1014 cm-3, the measurement accuracy is no worse than 1%, and the temporal resolution reaches 30 ns.